Railway signalling forms a critical part of the proper operation of a railway network. The signals provided to train drivers for example, enable them to properly negotiate travel from one point to the next. In this respect, the signals can provide information relevant to the speed of a train, the allowability of the train to pass through or enter a particular area (for example a train station), and they can alert the driver to upcoming signal position or danger in the event of unusual or altered traffic conditions or an unforeseen accident having occurred.
Trains are generally extremely heavy vehicles and thus require significant distance to slow or stop. For this reason, signals generated at one point often need to be visible to the train driver well in advance of that point so that the train driver can take appropriate action in the event that an alteration to the speed of the train or other characteristic needs to be undertaken. Due to the limited sighting times caused by the high speed at which trains travel, it is necessary to ensure very accurate alignment of a signal relative to the rail line. It follows that the light signals which are used in railway networks, in particular in country areas, are usually formed as a focused beam of high intensity light. Currently, both LED and incandescent lighting is employed.
Because signals are often required to be visible to a train driver over a significant distance, the orientation or direction of the signals can require a high degree of accuracy. Typically the alignment accuracy must be within ±2.5°. Accordingly, when railway signalling is set, it is common for the alignment procedure to be time consuming and consultative to ensure accuracy of alignment.
While the need for accuracy is usually most acute in relation to high speed or high load railway networks, it is also applicable to urban railway networks, although the reaction distances involved in urban railway networks are usually not as great as those involved in country railway networks. Nevertheless, the same principles apply in urban railway networks, that train drivers must have good visual communication with the signalling so that they are accurately informed at all times in relation to driving conditions.
A major difficulty with post mounted signals, such as is commonly used in railway in urban railway networks, is that the signals periodically require maintenance, such as for lens cleaning, repairs, refurbishment and for general upkeep. Activities of this kind are generally difficult to undertake, given that the signal units (hereinafter the “signal head”), is normally mounted well above the ground, thus requiring ladder or scaffold access. Ladder access is typically the method used to reach a signal head, but that carries with it certain workplace risks for the worker who is elevated well above the ground. It needs to be appreciated that the worker will often have to carry out difficult maintenance tasks, often requiring both hands, and this places that worker at risk of falling from the ladder. Furthermore, in electrified railways, the signals are often close to electrical hazard areas, often making maintenance tasks more difficult.
One solution to the above problem has been to develop posts that can tilt so that the signal head can be shifted from an elevated position to a position closer to the ground and away from electrical hazard areas and where a worker can access the signal head without requiring elevation by ladder or scaffold. While that form of post reduces the potential for injury to the worker, it also has certain disadvantages.
A first disadvantage is that tilting posts will position the signal head facing in a certain direction so that access to some parts of the signal head is good and other parts awkward. To alleviate that difficulty, some mounting arrangements can be loosened so that the signal head can be shifted relative to the post and access to other parts of the signal head is improved. However when the signal head is moved, existing mounting arrangements do not necessarily allow the accurate return of the signal head to its position prior to being moved. Thus, the body of persons previously engaged to align the signal head must be engaged again if the signal head is to be certified as having the accuracy it is required to have. This increases the cost of the operation of maintaining the signal head, and also provides significant inconvenience by having to assemble different groups of personnel when a signal head is to be maintained.
Another drawback with the above arrangement which allows the signal head to be rotated, is that the arrangements available to date have generally had the disadvantage that they can also allow the signal head to be inadvertently released from the end of the post, if care is not taken to ensure that the signal head does not slip relative to the post. Accordingly, in posts that employ a tilt portion, if care is not taken to ensure that the portion which is tilted does not tilt below a 90° angle to vertical, there is the possibility that the signal head could slip off the post, thus preventing the potential for injury to the workers in the immediate vicinity and potential damage to the signal head itself.
Applicant has identified that that an improvement in the mounting of signal heads to posts can provide better access to the signal head for maintenance purposes and can improve the accuracy of alignment of the signal head when the head is returned to an operating position following maintenance.